Plain English Meaning

The level you cruise at when flying IFR. ATC normally tells you what it is. If you are flying IFR in airspace where ATC is not controlling you, you pick the level using a rule based on the direction you are flying.

Full Definition

The altitude or flight level assigned to an aircraft operating under Instrument Flight Rules (IFR) during the en route, level-cruise portion of flight. In controlled airspace, the IFR cruising altitude is the one assigned by ATC. In uncontrolled airspace, the pilot selects an altitude in accordance with the IFR cruising altitude rule (14 CFR 91.179), which is based on magnetic course: odd thousands plus any assigned increment for 0° through 179°, and even thousands for courses 180° through 359°.